Revs stick at 3000 rpm

[Marbella]

when I take my foot off the gass pedel to change gear the revs do not drop under 3000 rpm. In the past a mechanic cleaned out the throtle body and for about 4 months it solved the problem. Any body any ideas why. taking the car back tothe garage every 4 months seems a little odd. The problem gets worse if I use normal unleaded. My car is an S40 T4